The present invention concerns both an operation diagnostic system for an angle detector designed to measure the angle of a rotating part, and the process by which the diagnosis is made.
Angle detectors are known in which the angle traveled by the rotating part is measured via electrical pulses transmitted to a processor designed to make use of that information, sometimes after electronic pre-processing of the signal by a digital-analog converter. This type of detector is used, for instance, in automotive steering columns, where it transmits to an on-board processor the angular position of the steering wheel with respect to a fixed position.
With this type of detector, speed and direction of rotation are measured by two sensitive components, often consisting of optic forks, which work with a second element such as a notched disk. Each of said sensitive components features a light emitter and a light receptor, positioned on either side of the notched disk, in the same axis. The light beam is alternately continuous and cut off, depending on whether the emitter-receptor pair faces a notch or an uncut sector of the disk. Thus, as the shaft rotates, the emitter generates a digital signal in the form of a pulsed output.
Two sensitive components are positioned such that the electrical signals generated are 90.degree. out of phase (.pi./2 radians), which allows the device to detect the direction of the shaft's rotation as well as its speed. This information is then transmitted to a processor, which uses it in whatever manner it has been designed for. In the case of a steering column, the main function of the processor is to calculate the rotation angle of the steering wheel.
To save costs, and for greater reliability, the output signal of the sensitive components is often carried to the processor by a single wire. Next, the two digital signals are encoded (e.g., a weighted encoding of their respective voltages) in order to produce an analog output signal which is easy to decode. The original signals are then simply reconstituted by comparing the single signal with predefined thresholds. This task is often carried out by the processor.
However, voltage levels fluctuate only if shaft rotation reaches a certain minimum angle, known as the blind angle. The blind angle is equal to one fourth of the period of the geometric configuration of the notched disk. This is not generally considered a problem, because the blind angle is small, and because applications take it into account.
However, when one of the elements of the device described above fails (emitter, receptor, coding or processing circuitry, electrical connection, etc.), the processor can be fooled by the lack of fluctuation in the detector's output signal into concluding that the shaft is no longer turning, or that its rotation is smaller than the blind angle.
Therefore, it has been desired to test the detector system and diagnose its real condition, so that an appropriate response can be made in case of need.
Detectors with diagnostic systems capable of running tests whenever the central processor issues the appropriate command are already available. However, existing assemblies are often costly, as they involve multiple electrical connections: one or two wires to carry the angle information, one to transmit the diagnostics command and one to send the test results to the central processor.
Other existing solutions use a single wire for transmitting all information, but they call for the multiplexing of signals, which means increased complexity and, again, higher costs. The cost of such solutions makes them unsuited to mass production applications such as in automobiles, where prices must be kept as competitive as possible. In particular, for automobile steering wheel applications the aforesaid type of solution is not useful.